1. Field of the Invention
The present invention relates to crystallization of silicon, and more particularly, to a laser beam pattern mask and a crystallization method using the same to improve the crystallization characteristics.
2. Discussion of the Related Art
With society becoming more dependent on information, the demand for various display devices has steadily increased. To meet such demands, efforts have been made recently to research flat panel display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), electro-luminescent displays (ELDs), vacuum fluorescent displays (VFDs), and the like. Practical applications of these flat panel display devices include integration with various appliances for display purposes.
In particular, LCDs have been used as a substitute for cathode ray tubes (CRTs) in association with mobile image display devices. Compared to CRTs, LCDs have superior picture quality, light weight, thin size, and low power consumption. Thus, LCDs are currently most widely used. Various applications of LCDs are being developed in association with not only mobile image display devices such as monitors of notebook computers, but also as monitors of televisions to receive and display broadcast signals, and as monitors of desktop computers.
Such LCDs mainly include a liquid crystal panel for displaying an image and a driver for applying a drive signal to the liquid crystal panel. The liquid crystal panel includes first and second substrates joined together with space therebetween and a liquid crystal layer constituted by liquid crystals sealed in the space provided.
The first substrate (TFT array substrate) includes a plurality of uniformly spaced gate lines arranged in one direction and a plurality of uniformly spaced data lines arranged in a direction perpendicular to the gate lines. The first substrate also includes a plurality of pixel electrodes arranged in the form of a matrix array at respective pixel regions, each pixel region defined by an intersection of each gate line and each data line. The first substrate further includes a plurality of thin film transistors (TFTs), each of which is switched on by a signal on an associated one of the gate lines and transmits a signal on an associated one of the data lines to an associated one of the pixel electrodes.
The second substrate (color filter substrate) includes a black matrix layer for blocking incidence of light to a region other than the pixel regions. The second substrate also includes red (R), green (G), and blue (B) color filter layers for reproducing color tones and a common electrode for reproducing an image.
The first and second substrates are joined by a seal member having a liquid crystal injection port formed by spacers to maintain a certain amount of space between the first and second substrates. Liquid crystal material is sealed in the space between the first and second substrates.
The driving principle of the above-mentioned general LCDs utilizes optical anisotropy and polarization properties of the liquid crystals. Since liquid crystals have thin and elongated molecular structures, molecules thereof have an orientation in a certain direction. It is possible to control the orientation of liquid crystal molecules by intentionally applying an electric field to the liquid crystal molecules. By controlling the orientation of liquid crystal molecules, the arrangement of liquid crystal molecules is varied so as to exhibit optical anisotropy. Images are generated when light incident to the liquid crystals is refracted in the direction in which the liquid crystal molecules are oriented.
Currently, active matrix LCDs, in which thin film transistors and pixel electrodes connected to the thin film transistors (TFTs) are arranged in a matrix array, are of most interest because they offer superior resolution and superior moving picture reproduction capabilities. Because the semiconductor layer of each TFT in LCDs is made of polysilicon, it is possible to form TFTs and driving circuits on the same substrate. Doing so eliminate the need to use a separate process for connecting the TFTs and driving circuits. Therefore, the manufacturing process of such LCDs is simplified. In addition, polysilicon has rapid response because the field effect mobility of polysilicon is 100 to 200 times that of amorphous silicon. Polysilicon also exhibits superior stability to temperature and light.
Formation of polysilicon may be classified into a low-temperature process and a high-temperature process in terms of process temperature. The high-temperature process requires a temperature condition higher than the strain temperature of an insulating substrate on which polysilicon is to be grown. The process temperature of the high-temperature process is on the order of 1,000° C. For this reason, one disadvantage is that it is necessary to use expensive, high thermal resistance substrates such as quartz in place of cheap, low heat resistance substrates such as glass as the insulating substrate.
Furthermore, a high surface roughness is required to form a polysilicon thin film using the high-temperature process. However, the same high-temperature process degrades the crystallinity of fine crystalline grains. For this reason, another disadvantage of polysilicon formed in accordance with the high-temperature process is a degradation in the very characteristics required for the application thereof, as compared to polysilicon formed in accordance with the low-temperature process. To this end, research into development of polysilicon formation techniques is currently being conducted where amorphous silicon, capable of being deposited at a low temperature, is crystallized into polysilicon.
Laser annealing uses a method for irradiating a pulse-shaped laser beam to a substrate. In accordance with this method, melting and solidifying amorphous silicon are repeated at intervals of 10 to 102 nanoseconds by the pulse-shaped laser beam. This technique minimizes damage to a lower insulating substrate. Because of such an advantage, laser annealing has been the most preferred technique for the low-temperature crystallization process.
Hereinafter, a silicon crystallization method using the above-mentioned laser annealing will be described in conjunction with the annexed drawings. FIG. 1 is a graph depicting the grain size of amorphous silicon depending on the density of laser energy. As shown in FIG. 1, crystallization of amorphous silicon may be classified into three regions in accordance with the intensity of laser energy.
The first region is a partial melting region, in which laser energy is irradiated on an amorphous silicon layer at an intensity capable of melting only the surface of the amorphous silicon layer. In the first region, the surface of the amorphous silicon layer is partially melted in accordance with the irradiation. After solidification, small-sized crystal grains are formed at the surface of the amorphous silicon layer.
The second region is a near-complete melting region, in which laser energy is irradiated to an amorphous silicon layer at an intensity higher than that of the first region capable of almost completely melting the amorphous silicon layer. Small nuclei remaining after the melting are used as seeds to grow the crystals. Although it is possible to grow more crystal grains in the second region as compared to the first region, it is difficult to obtain homogenous crystal grains. Furthermore, the second region is considerably narrow, as compared to the first region.
The third region is a complete melting region, in which laser energy is irradiated to an amorphous silicon layer at an intensity higher than that of the second region capable of completely melting the amorphous silicon layer. In this case, solidification occurs after the amorphous silicon has been completely melted. Accordingly, homogeneous nucleation of crystals is possible. After the irradiation, a crystalline silicon layer consisting of homogenous fine crystal grains is formed.
In the process of producing polysilicon using the laser density of the second region, the number of times the laser beam is irradiated and the laser beam overlap ratio are controlled to enable formation of homogenous large crystal grains. However, the boundaries of crystal grains of polysilicon obstruct current flow. For this reason, it is difficult to obtain a reliable thin film transistor device. Furthermore, an insulating film over which polysilicon is grown becomes damaged due to current induced by collision of electrons in the crystal grains of the polysilicon. The result is a product having poor quality.
In order to solve this problem, sequential lateral solidification (SLS) technique was proposed for forming single-crystalline silicon. This technique is based on the fact that silicon crystal grains are grown at the boundary between liquid-phase silicon and solid-phase silicon in a direction perpendicular to the boundary (Robert S. Sposilli, M. A. Crowder, James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956˜957, 1997). In accordance with the SLS technique, it is possible to crystallize amorphous silicon into single crystals of 1 μm or greater by appropriately adjusting the laser energy intensity, irradiation range, and translation distance of a laser beam irradiated on the amorphous silicon such that silicon crystal grains are laterally grown to a certain length. One draw back is that an irradiation device typically used in such an SLS process cannot change amorphous silicon deposited over a large-area substrate into polycrystalline silicon at one time because the typical irradiation device concentrates a laser beam on a narrow region of the substrate. To this end, a substrate deposited with an amorphous silicon layer is mounted on a movable stage. After a laser beam irradiates a certain region of the substrate, the substrate is repositioned by moving the stage to allow the laser beam to irradiate the next region of the substrate. In such a manner, the entire region of the substrate is irradiated by the laser beam.
FIG. 2 is a schematic view illustrating a general SLS irradiation device. As shown in FIG. 2, the SLS irradiation device includes a laser generator for generating a laser beam, an attenuator 1 for controlling the energy intensity of the laser beam, a first mirror 2a for changing the traveling path of the laser beam, a telescopic lens 3 for diverging the laser beam, and a second mirror 2b for again changing the traveling path of the laser beam. The SLS irradiation device also includes a lens 4 for homogenously condensing the laser beam, a third mirror 2c for again changing the traveling path of the laser beam, and a field lens 5 for changing the shape of the laser beam to an appropriate shape enabling the laser beam to be incident to a laser beam pattern mask 6. The laser beam pattern mask 6 has a predetermined pattern allowing selective transmission of the laser beam. The SLS irradiation device further includes a projection lens 7 for reducing the scale of the laser beam passing through the laser beam pattern mask 6 to a predetermined scale and irradiating the same onto substrates.
The laser generator is an excimer laser typically employing XeCl (308 nm) or KrF (248 nm). The laser generator emits an unprocessed laser beam, which is converted into a predetermined laser beam pattern by sequentially passing through the attenuator 1, the mirrors 2a, 2b, and 2c adapted to change the traveling path of the laser beam, the lenses 5 and 7 having particular functions, and the laser beam pattern mask 6. The processed laser beam having the predetermined pattern is then irradiated on substrate 10.
Mirrors 2a, 2b and 2c are used to minimize the space occupied by the SLS irradiation device. In some cases, the space occupied by the SLS irradiation device may be increased or reduced by adjusting the number of mirrors.
A movable stage 8, on which a substrate 10 deposited with an amorphous silicon layer is mounted, is arranged at a position corresponding to the laser beam pattern mask 6. The stage 8 may be an X-Y stage. Since the substrate 10 is mounted on the stage 8 after fabrication, means for fixing the substrate 10 on the stage 8 is provided to prevent the substrate 10 from moving in relation to the stage 8 during the crystallization process. Crystallization of the entire region of the substrate 10 is achieved by moving the stage 8 in the X and Y directions such that the crystallized region of the substrate 10 is gradually increased.
The laser beam pattern mask 6 is divided into transmission portions, through which the laser beam passes, and shield portions, which block transmission of the laser beam. The width of each transmission portion determines the lateral growth length of crystal grains formed in one exposure.
Hereinafter, the laser beam pattern mask 6 will be described in detail with reference to the annexed drawings. FIG. 3 is a plan view illustrating a laser beam pattern mask used in a conventional laser irradiation process. FIG. 4 is a crystallized region formed in one shot of laser beam irradiated using the laser beam pattern mask of FIG. 3.
As shown in FIG. 3, the laser beam pattern mask, which is used in the conventional laser irradiation process, includes transmission portions A each providing an opened pattern having a first width a, and shield portions B each providing a shielded pattern having a second width b. The transmission portions A and shield portions B are alternately arranged.
Laser irradiation using the laser beam pattern mask is carried out as follows. A single shot of laser beam is irradiated on a substrate deposited with an amorphous silicon layer through the laser beam pattern mask 6 positioned above the substrate. At this time, the laser beam is irradiated on areas 22 (FIG. 4) corresponding to respective transmission portions A of the laser beam pattern mask 6. As a result, the portions of the amorphous silicon layer corresponding to respective irradiated areas 22 are melted into a liquid phase, as shown in FIG. 4. In this case, the intensity of laser energy used is in the range of the complete melting region capable of completely melting the amorphous silicon layer in the irradiated areas 22.
The region in which the substrate is irradiated by the laser beam in one shot is referred to as a “unit region” 20. The unit region 20 is defined to have a horizontal length L and a vertical length S as shown in FIG. 3.
Following the laser beam irradiation, silicon grains 24a and 24b are grown in each irradiated areas 22. The silicon grains 24a and 24b grow laterally toward boundary surfaces 21a and 21b. The boundary surfaces 21a and 21b are formed between the solid-phase amorphous silicon area and the liquid-phase area where the amorphous silicon layer has completely melted into a liquid phase. The lateral growth of the silicon grains 24a and 24b is in a direction perpendicular to the boundary surfaces 21a and 21b. 
When the width of the irradiated areas 22 corresponding to each transmission portion A is less than two times the growth length of the silicon grains 24a and 24b, the silicon grains 24a and 24b, which are grown inside of an area defined between the boundary surfaces 21a and 21b in a direction perpendicular to the boundary surfaces 21a and 21b, come into contact with each other during the growth. This contact stops the growth of silicon grains 24a and 24b. 
In order to grow more silicon grains, the stage on which the substrate is mounted is subsequently moved to allow unexposed areas of the substrate adjacent to the irradiated areas 22 to be irradiated. Subsequent irradiations allow crystal grains to grow connected to the crystal grains formed in the previous irradiation. Generally, the growth length of crystal grains connected to the previously irradiated areas is determined by the width of each transmission portion A and the width of each shield portion B in the laser beam pattern mask 6.
FIG. 5 is a plan view illustrating sequential processing steps of the conventional crystallization process. The conventional silicon crystallization process proceeds on the substrate 10 in the order shown in FIG. 5. That is, the irradiation position on the substrate 10 is horizontally translated from right to left (along the X-axis in the negative direction) by a distance corresponding to the length L of each transmission portion A of the laser beam pattern mask 6 as shown in FIG. 3 (translation {circle around (1)}). Translation {circle around (1)} is repeated after each crystallization process until the crystallization process is completed for a region corresponding to the horizontal length of the substrate 10.
Thereafter, the irradiation position on the substrate 10 is vertically translated from the upper side to the lower side (along the Y-axis in the negative direction) by a distance corresponding to ½ of the total width of one transmission portion A and one shield portion B in the laser beam pattern mask 6 ((a+b)/2) (translation {circle around (2)}) and the crystallization process is carried out. Subsequently, the irradiation position on the substrate 10 is again horizontally translated from to right (along the X-axis in the positive direction) by a distance corresponding to the length L of each transmission portion A of the laser beam pattern mask 6 (translation {circle around (3)}) and the crystallization process is carried out. The translation {circle around (3)} is repeated after every crystallization process until the crystallization process is completed for a region corresponding to the horizontal length of the substrate 10. The crystallization process associated with the translation {circle around (2)} and translation {circle around (3)} is carried out for unirradiated areas in each unit region 20 of the substrate 10 irradiated in the crystallization process associated with translation {circle around (1)}. As described above, the unit region 20 corresponds to the size of the laser beam pattern mask 6. The irradiated areas in the crystallization process associated with translation {circle around (2)} and translation {circle around (3)} are partially overlapped with the irradiated areas in the crystallization process associated with translation {circle around (1)} so as to connect the growth of crystals connected to the crystals formed in the previous crystallization process associated with translation {circle around (1)}.
Thereafter, the irradiation position on the substrate 10 is vertically translated from the upper side to the lower side (in a negative Y-axis direction) by a distance corresponding to the vertical length S of the laser beam pattern mask 6 (translation {circle around (4)}). After translation {circle around (4)}, the procedures associated with translations {circle around (1)} to {circle around (4)} are repeated to complete crystallization for the entire surface of the substrate 10.
The translation of the irradiation positions on the substrate 10 is achieved by moving the stage 8 (in FIG. 2), on which the substrate 10 is mounted, in relation to the laser beam pattern mask 6, which is fixed. The movement of the stage 8 is carried out in a direction opposite to the directions shown for translations {circle around (1)} to {circle around (4)} in FIG. 5.
FIGS. 6A and 6B are scanning electron microscope (SEM) micrographs showing polysilicon grown in accordance with the conventional crystallization method. When a polysilicon layer is formed in accordance with the crystallization method of FIG. 5 using the conventional laser beam pattern mask, an overlap region is formed between adjacent crystallized areas or patterns. This overlap region can be macroscopically observed in the form of a line, in contrast to other crystallized areas. The overlap region can be macroscopically observed in the form of a line because laser beam irradiation is carried out two or more times for the same region in the typical crystallization method of FIG. 5. That is, such a linear overlap region is created when laser beam irradiation on the substrate is carried out using a laser beam pattern mask in which a plurality of linear transmission portions having a certain width are uniformly spaced apart from one another by a certain distance.
In this case, the overlap regions of the crystallized patterns are macroscopically observed in the form of a plurality of spaced lines. These overlap regions serve as grain boundaries. When an active element is arranged on such an overlap region, the performance characteristics of the element are reduced because the mobility of elections at the grain boundaries are affected.
FIG. 6B is a SEM micrograph in the case in which the length of crystallized grains is smaller than that of FIG. 6A. In either case of FIG. 6A or FIG. 6B, a plurality of lines serving as grain boundaries were observed in the overlap regions of the crystallized patterns. Thus, the above-mentioned conventional laser beam pattern mask and the conventional crystallization method using the conventional laser beam pattern mask have various problems.
First, when laser beam patterns are irradiated on a substrate through the conventional laser beam pattern mask in a crystallization process, irradiation overlap is generated between adjacent laser beam patterns. In particular, the irradiation of the laser beam patterns is carried out through the linear transmission portions of the laser beam pattern mask. In this case, linear overlap regions are formed between adjacent irradiated areas. Normal crystallization cannot be achieved in the linear overlap regions, so the linear overlap regions create linear grain boundaries. These linear grain boundaries are generated in a regular and repeated pattern.
If crystallization involving repeated formation of such linear grain boundaries is effectuated at pixel regions on the substrate, different signal lines on the substrate may interfere with each other. Such interferences may be observed in the form of wavy image patterns, such as a moire pattern.
Second, when crystallization involving repeated formation of the above-mentioned linear grain boundaries is effectuated at regions corresponding to drivers on the substrate, the possibility of grain boundaries being formed in the channels of active elements increases. As a result, the reliability of the elements may be greatly affected.